In mammals, the initial detection of sensory information, such as tactile, proprioceptive, thermal, and pain stimuli from both external objects and the body, is carried out by primary sensory neurons. The cell bodies of these neurons locate in dorsal root ganglia (DRG). Their axons are able to innervate specific targets in the periphery and the spinal cord. The long-term objective of our research is to understand the molecular mechanism of how the projection patterns of primary sensory neurons are established and to determine the neural circuits of different subpopulation of sensory neurons. Recently, we have identified a large family of G protein coupled receptors (GPCRs), called Mrgs, expressed specifically in primary sensory neurons in DRG. Interestingly, different Mrgs are expressed in different subpopulations of sensory neurons. We have generated several knock-in mouse lines, in which different axonal tracers including farnesylated green fluorescent protein and human placenta alkaline phosphatase were inserted into different Mrg loci. By analyzing these mice, we found that sensory neurons expressing different Mrgs have highly distinct and specific projection patterns both in the periphery and the spinal cord. One striking example is that the axons of MrgB4+ neurons only innervate hairy skin but not glabrous skin. To our knowledge, MrgB4 is the first marker which can segregate hairy skin innervating neurons from glabrous skin innervating neurons. Interestingly, most of MrgB4+ fibers terminate at a specific region of hair follicles. Furthermore, the projection of MrgB4+ axons in the dorsal horn of the spinal cords is also very unique. The MrgB4+ fibers terminations in the dorsal horn lamina layer form an unusual discontinuous band, suggesting that MrgB4+ fibers converge to specific regions within the lamina layer. The high specificity of MrgB4+ axon projection pattern raises the question of whether this GPCR play a role in the establishment of such specific pattern in the periphery and the spinal cord. Here we propose to study the function of MrgB4 in axon guidance of sensory neurons and their neural circuits in the higher order of the CNS. Aim I is to analyze the projection patterns of MrgB4+ fibers in hairy skin and spinal cord of MrgB4 homozygous mutant mice, from which we would like to determine whether MrgB4 plays roles in axon guidance or target recognition. In Aim II, we will determine whether hair growth is involved in establishment and maintenance of sensory innervations in hairy skin. To achieve this goal, we will study the projection patterns of MrgB4+ fibers in hairless knockout mouse background. Aim III we would like to map the neural circuit of MrgB4+ neurons using a trans-neuronal anterograde tracer, wheat germ agglutinin (WGA). We will generate MrgB4-WGA knock-in mice and we will carry out a detailed analysis of the pattern of WGA expression to identify the second- (possibly third-) order neurons to MrgB4+ primary sensory neurons labeled by WGA. We believe that studying the molecular mechanism of how sensory nerves establish proper projections in the skin and the spinal cord will have broad implications for understanding neuropathological diseases associated with abnormal wiring in the somatosensory system such as those in certain chronic conditions. In addition, the study of sensory neural circuits will greatly facilitate our understanding of how sensory information is transmitted and processed, which will be essential for developing novel drugs to treat diseases with abnormal sensory function such as chronic pain. [unreadable] [unreadable]